A. Field of the Invention
The present invention relates to a method of and system for driving a PDP (Plasma Display Panel) and, more particularly, to a method of and system for driving a PDP which is designed to increase the amount of data processed in unit time.
B. Description of Prior Art
In general, a PDP performs an electric discharge by regulating the voltages applied between the vertical and horizontal electrodes of cells that constitute a pixel, and the amount of light discharged can be controlled by varying the period of time for performing a discharge in the cell.
A picture is formed when the vertical and horizontal electrodes of each cell receive a write pulse for feeding digital picture signals, a scan pulse for performing a scan, a sustain pulse for sustaining a discharge, and an erase pulse for suspending the discharge of the cell being discharged, wherein the pulses are driven in the matrix form.
Grey level by steps that is needed to display the whole picture can be realized by varying the period of time for performing a discharge of each cell in a predetermined time required to display the whole picture (i.e., 1/30 seconds for the NTSC mode TV signals).
The luminance of a picture is dependent on the grey level at the time that each cell is driven to the maximum. To increase the luminance, the driving circuit has to be designed to sustain the time required for a discharge in the cell within a required time to display the picture to the utmost.
The contrast, which is the degree of difference between the lightest and darkest parts of a picture, is dependent on the grey level and luminance of the background lighting. To increase the contrast, the background has to be dark with the increase of the luminance.
The flat panel display of an HD TV needs 256 grey levels, the resolution of more than 1280.times.1024, and the contrast of above 100:1 under background luminance of 200 lux luminance.
To display a picture with 256 grey levels, each of digital RGB picture signals has to be an 8-bit signal. The period of time for performing a discharge in each cell must be sustained to the utmost in order to reach the required luminance and contrast.
Line scanning or subfield scanning is used as a method of realizing grey levels. For PDP applications, the subfield scanning has received most attention lately.
In the subfield scanning, 8-bit picture signals are gathered in groups of the same weight bits, from the MSB (Most Significant Bit) to the LSB (Least Significant Bit). The most significant bit is scanned for time T and the lower bits are each scanned for ##EQU1##
in order of vicinity to the MSB, so that subfields are formed. 256 grey levels are then realized by using the eyes' integration effect for the lights emitted from the respective subfields.
However, the PDPs, which must be driven in a matrix format, has a disadvantage in that the write pulses cannot be applied to two or more horizontal electrodes at a time with respect to a given vertical electrode, and the horizontal electrodes have to be driven at different times.
Thus, the time required for scanning all horizontal electrodes is needed in forming each subfield so that each cell is kept discharged only for the time shortened by the time required for a scanning from an average time for scanning the respective subfields.
Further, a discharge cannot be sustained during the scanning time that increases with the number of horizontal electrodes, thus deteriorating the luminance and contrast of the PDP. Thus, the scanning time must be as short as possible.
Since the difference between the periods of a discharge of the upper and lower bits is great and the subfields are sequentially constructed, a flicker phenomenon occurs much. To avoid the flicker phenomenon, it is needed to construct the upper bit subfields, taking much time for a discharge, and the lower bit subfields taking a short time for a discharge in appropriate order.
FIG. 1 illustrates a cell structure of a three-electrode surface discharge AC PDP that is widely used now.
As shown in FIG. 1, lower and upper insulating plates 1 and 2 are supported in parallel by separation walls 10 for separating the cells. Row electrodes 3 having one scan electrode and one common electrodes are arranged in parallel with one another on the lower insulating plate 1.
Column electrodes 4 arranged in parallel with one another under the upper insulating plate 2 form a matrix with the row electrodes 3.
Lower and upper insulating layers 5 and 6 cover the row electrodes 3 and the column electrodes 4, respectively, for the protection purpose, so that a discharge driven by the DC voltage applied between the electrodes becomes extinct immediately.
To sustain a discharge in an AC PDP with the electrode structure as described above, an AC voltage which is successively inverted in the polarity has to be applied between the electrodes.
Protection layer 7 is formed on the lower insulating layer 5 and is made from MgO thin films, thus prolonging the life of the insulating layer 5, enhancing the emission efficiency of secondary electrons, and preventing the change in the discharge characteristic that might be caused by oxide contaminants of ignited metals.
Phosphor 9 is formed on the upper insulating layer 6 and excited by ultraviolet rays emitted during an electric discharge, emitting red, green and blue visible rays.
Discharge space 8, a space for an electric discharge in the cell is usually filled with mixed gases of Ar and Xe in order to enhance the efficiency of the ultraviolet ray emission.
FIG. 2 shows the electrode arrangement of a general three-electrode surface discharge AC PDP.
As shown in FIG. 2, each cell 11 is positioned at an intersection of the row and column electrodes. The row electrodes has a group of scan electrodes S.sub.1 to S.sub.m for scanning a field, and a group of common electrodes C.sub.1 to C.sub.m for sustaining an electric discharge. The column electrodes are generally used to apply data.
Sealing region 12 maintaining the vacuum state inside the PDP is defined by the separation walls formed between the insulating plates 1 and 2, thus closing and securing the PDP's edges with a sealing material.
FIG. 3 is a waveform diagram of driving pulses that are used in a general three-electrode surface discharge AC PDP.
As shown in FIG. 3, a sustain pulse A for sustaining a discharge of the cell is applied to the common electrodes C.sub.1 to C.sub.m, while another sustain pulse B that is same in the form as the pulse of the common electrode but different in position from it is applied to the scan electrodes S.sub.1 to S.sub.m.
Each of the scan electrodes S.sub.1 to S.sub.m also receives scan pulses for scanning a field and erase pulses for suspending a discharge of the discharged cell, thus controlling the switching operation of the cell.
Column electrodes D.sub.1 to D.sub.m receive data pulses synchronized with the scan pulses applied to the scan electrodes, generating write pulses. When data pulses of the positive (+) polarity is applied to the electrode D.sub.1 and scan pulses in synchronization with the data pulse are transferred to the cell S.sub.1, the voltage between the electrodes S.sub.1 and D.sub.1 exceeds the threshold voltage that causes an electric discharge in the cell.
Such a discharge will be sustained until the next erase pulse by the electric field formed from the particles that are electrically discharged in the insulating layers during the discharge and by the electric field formed by the sustain pulses of electrodes S.sub.1 and C.sub.1. If the amplitude of the erase pulse is smaller than that of the sustain pulse, an electric discharge occurs a little to such a degree that the sum of the electric fields caused by the charged particles and the erase pulse cannot sustain the discharge.
To summarize the functions of the respective electrodes described above, the scan electrodes are used to sustain an electric discharge and scan a field, while the common electrodes can only sustain a discharge. The data electrodes are in charge of receiving data for constructing a field.
FIG. 4 is a waveform diagram of driving pulses according to the prior art, showing the voltage between the scan and common electrodes when the waveforms in FIG. 3 are applied to the cell electrodes.
The waveforms shown in FIG. 4 can be obtained by combining the inverted waveforms of the scan electrodes, based on the waveforms of the common electrodes.
This basic driving waveform is characterized in that the scan pulse can be applied in half a period of the sustain pulse because it appears once in a period of the sustain pulse.
FIG. 5 illustrates a scanning based on the conventional subfield driving method of realizing 256 grey levels.
A field is composed of 8 subfields each of which has a constant subfield time T.sub.A. The time T.sub.FIELD that is needed to construct a field amounts to 8T.sub.A. The time used to perform a discharge out of the subfield time T.sub.A is determined as ##EQU2##
in order from the MSB to the LSB. Thus the time T.sub.S that is available to a discharge of the time 8T.sub.A for constructing a field will be 2T.sub.A. The time T.sub.NS that cannot be used to perform a discharge is 6T.sub.A. ##EQU3##
The percentage of waste time T.sub.NS is 75%, calculated as
The efficiency is calculated as ##EQU4##
These values show us that the time that can be used for a discharge is actually not more than 25% of the total time in an AC PDP using the subfield driving method, so that the luminance is greatly deteriorated.